Self-attaching female fasteners, including pierce and clinch nuts, formed by cold header techniques and secondary press operations generally include a central pilot portion having a bore therethrough, an annular end face surrounding the bore, a flange portion surrounding the pilot portion including an end face, and an annular groove in the end face of the flange portion. The annular groove includes an annular inner side wall adjacent the pilot portion, a bottom wall spaced below the plane of the annular end face of the flange portion and an outer side wall extending from the bottom wall to the end face of the flange portion. To improve torque resistance or prevent rotation of the self-attaching female fastener on a panel following installation, the bottom wall of the groove may also include protuberances or ribs, including circular protuberances spaced from the inner and outer side walls of the groove, as disclosed, for example, in U.S. Pat. No. 5,531,552 assigned to the predecessor in interest of this application. U.S. Pat. No. 5,549,430, also assigned to the predecessor in interest of the assignee of this application, discloses a self-attaching nut of this type, wherein the bottom wall of the groove includes a plurality of spaced arcuate or semi-circular protrusions integral with the pilot portion. The prior art also includes radial ribs integral with the bottom wall of the groove and the inner and outer side walls, wherein the inner side wall of the annular groove or the outer wall of the pilot portion is inclined from the bottom wall radially outwardly. However, this self-attaching nut does not have sufficient retention on the panel for many applications. Finally, the prior art also includes self-attaching nuts having a “dovetail-shaped” annular groove, including small triangular protuberances integral with the bottom wall and the inner side wall and the bottom wall and the outer side wall. However, these small radial protuberances do not provide sufficient torque resistance for many applications.
There is therefore a need for a self-attaching fastener, particularly a pierce nut, having improved torque resistance and push-off strength. The self-attaching female fasteners of this invention provide these improvements as described below.